The present invention relates to a positioning device for an actuator; and more particularly but not exclusively to positioning a transducer in relation to a magnetic recording disk.
The art of magnetic disk recording is well established. These disks are used in computer technology and serve as rapid access memory extensions. A disk unit, sometimes called a disk drive, is comprised of one or more coaxial disks, each having one or two magnetic surfaces for recording data in concentric tracks. When more than one surface is involved, one also speaks of concentric cylinders, the center of a track being the geometric line of intersection of a plane (the recording surface) with a cylinder. Each surface cooperates with one particular transducer to be positioned above any of the tracks/cylinders. The disk or disks spin as the transducer remains stationary for recording data on or recording data from the particular track. A track change requires the transducer to be moved radially in respect to the spin axis by means of an actuator. The present invention relates particularly to such an actuator.
Such actuators and transducer positioners are varied in design and performance, and cost tradeoffs usually determine their choice in any particular instance. One type of actuators is called "a voice coil motor." More recently, a stepper motor has been employed having a capstan to which is connected a split band having two ends which are fastened to a carriage member for the transducer. This device translates the rotational motion of the motor into a linear one. The band is tensioned so that any rotational step of the motor, regardless of direction, is translated into an incremental, linear motion of the transducer-carrying member. This type of drive has been used successfully; and the present invention is directed towards improvements thereof, bearing in mind that the principle involved, namely, translating rotational steps into linear displacement and positioning steps, can be applied also in other fields.
The presently used split-band-positioning devices have certain limitations. First of all, it is inherent that the capstan cannot rotate by more than 360.degree., i.e., it cannot make a full turn. For practical purposes, the angular range is limited to about 300.degree.. The stepping motors themselves are limited to a certain angle per step, this being today about 1.8.degree. per step for a total of about 167 steps. This means, such a positioning device can access at the most about 167 cylinders or tracks. This number presents a severe limitation. Moreover, the linear displacement per step is directly related to the diameter of the capstan. A 1.8.degree. angular step will result in a particular, linear displacement for a given capstan diameter. That linear displacement step is directly equal to the maximum possible radial distance between adjacent tracks on the recording disk. Thus, if track density is to increased (a modern trend), the capstan diameter must decrease. That diameter determines directly the looping diameter for the split band; if it is too small, severe stress problems arise.